| Scheelite | |
|---|---|
 | |
| General | |
| Category | Tungstate mineral |
| Formula | CaWO4 |
| IMA symbol | Sch[1] |
| Strunz classification | 7.GA.05 |
| Crystal system | Tetragonal |
| Crystal class | Dipyramidal (4/m) H-M symbol: (4/m) |
| Space group | I41/a |
| Unit cell | a = 5.2429(3), Å c = 11.3737(6) Å;Z = 4 |
| Identification | |
| Color | Colorless, white, gray, dark brown, brown, tan, pale yellow, yellow-orange, golden yellow, pale shades of orange, red, green, etc.; colorless in transmitted light and may be compositionally color zoned |
| Crystal habit | Pseudo-octahedra, massive, columnar, granular |
| Twinning | Common, penetration and contact twins, composition plane {110} or {001} |
| Cleavage | On {101}, distinct; on {112}, interrupted; on {001}, indistinct |
| Fracture | Subconchoidal to uneven |
| Tenacity | Brittle |
| Mohs scale hardness | 4.5–5 |
| Luster | Vitreous to adamantine |
| Streak | White |
| Diaphaneity | Transparent to opaque |
| Specific gravity | 5.9–6.1 |
| Optical properties | Uniaxial (+) |
| Refractive index | nω = 1.918–1.921,nε = 1.935–1.938 |
| Birefringence | δ = 0.017 |
| Pleochroism | Definite dichroic in yellow (yellow to orange-brown) |
| Fusibility | With difficulty |
| Solubility | Soluble in alkalis. Insoluble in acids |
| Other characteristics | Fluorescence under short-wave UV is bright blue, bluish white to yellow. Specimens with more molybdenum tend to fluoresce white to yellow, similar to powellite. Occasionally, it fluoresces red under mid-wave UV. |
| References | [2][3][4][5] |
Scheelite is acalcium tungstatemineral with thechemical formulaCaWO4. It is an importantore oftungsten (wolfram). Scheelite is originally named after Swedish chemistCarl Wilhelm Scheele (1742–1786). Well-formedcrystals are sought by collectors and are occasionally fashioned intogemstones when suitably free of flaws. Scheelite has beensynthesized using theCzochralski process; the material produced may be used toimitate diamond, as ascintillator, or as asolid-statelasing medium. It was also used inradium paint in the same fashion as waszinc sulphide, andThomas Edison invented afluoroscope with a calciumtungstate-coated screen, making the images six times brighter than those withbariumplatinocyanide; the latter chemical allowedRöntgen to discoverX-rays in early November 1895. The semi-precious stone marketed as 'blue scheelite' is actually a rock type consisting mostly of calcite and dolomite, with occasional traces of yellow-orange scheelite.
Its crystals are in thetetragonalcrystal system, appearing as dipyramidal pseudo-octahedra. Colors include golden yellow, brownish green to dark brown, pinkish to reddish gray, orange and colorless. Transparency ranges from translucent to transparent, and crystal faces are highlylustrous (vitreous to adamantine). Scheelite possesses distinctcleavage, and its fracture may besubconchoidal to uneven. Itsspecific gravity is high at 5.9–6.1 and itshardness is low at 4.5–5.[2] Aside from pseudo-octahedra, scheelite may be columnar, granular, tabular or massive inhabit.Druzes are pretty rare and occur almost exclusively at Zinnwald,Czech Republic.Twinning is also commonly observed, and crystal faces may be striated. Scheelite has a whitemineral streak and is brittle.
Gems cut from transparent material are fragile. Scheelite'srefractive index (1.918–1.937 uniaxial positive, with a maximumbirefringence of 0.016) anddispersion (0.026) are both moderately high. These factors combine to result in scheelite's high lustre and perceptible "fire", approaching that ofdiamond.
Scheelitefluoresces under shortwaveultraviolet light, the mineral glows a bright sky-blue. The presence ofmolybdenum trace impurities occasionally results in a green glow. Fluorescence of scheelite, sometimes associated with native gold, is used by geologists in the search for gold deposits.
Scheelite occurs incontact metamorphicskarns; in high-temperaturehydrothermalveins andgreisen; less commonly ingranitepegmatites.[2] Temperature and pressure of formation is between 200 and 500 °C (400 and 900 °F) and from 200 to 1,500 bars (2,900 to 21,800 psi).[7] Typical mineral association includescassiterite,wolframite,topaz,fluorite,apatite,tourmaline,quartz,grossular–andradite,diopside,vesuvianite andtremolite.[2]
Scheelite usually occurs in tin-bearing veins and is sometimes found in association with gold. Fine crystals have been obtained from Caldbeck Fells inCumbria, Zinnwald/Cínovec andElbogen inBohemia,Guttannen inSwitzerland, theGiant Mountains inSilesia,Dragoon Mountains inArizona and elsewhere. AtTrumbull in Connecticut andKimpu-san in Japan, large crystals of scheelite completely altered to wolframite have been found: those from Japan have been called “reinite.”[8] It was mined until 1990 atKing Island, Australia,Glenorchy inCentral Otago andMacraes Flat inNorth Otago and also at The Golden Bar mine at Dead Horse Creek during World War I inNelson, New Zealand. There is a high concentration of Scheelite in the Northeast of Brazil, mainly in theCurrais Novos mine in Rio Grande do Norte State.[9] One of the world's largest Scheelite mining companies is inLuoyang, China.[10]
Scheelite was first described in 1751 for an occurrence inMount Bispbergs klack,Säter,Dalarna,Sweden, and named forCarl Wilhelm Scheele (1742–1786).[3] Owing to its unusual heaviness, it had been given the nametungsten by the Swedes, meaning “heavy stone.” The name was later used to describe the metal, while the ore itself was given the name scheelerz or scheelite.[11]
 | This sectiondoes notcite anysources. Please helpimprove this section byadding citations to reliable sources. Unsourced material may be challenged andremoved.(April 2024) (Learn how and when to remove this message) |
Scheelite as a diamond imitation has been surpassed by more convincing products, likecubic zirconia andmoissanite. Synthetic scheelite is occasionally offered as natural scheelite, and collectors may thus be fooled into paying high prices for it.Gemologists distinguish natural scheelite from synthetic material mainly by microscopic examination: Natural material is very seldom without internal growth features and inclusions (imperfections), while synthetic material is usually spotless. Distinctly artificial curved striae and clouds of minute gas bubbles may also be observed in synthetic scheelite.
The visibleabsorption spectrum of scheelite, as seen by a hand-held (direct-vision)spectroscope, may also be of use: most natural stones show several faint absorption lines in the yellow region of the spectrum (~585 nm) due topraseodymium andneodymium trace impurities. Conversely, synthetic scheelite is often without such a spectrum.
Scheelite is widely used inphosphors,[12] particularly inscintillators for X-ray and gamma-ray detection.[13] The second and third iterations of theCryogenic Rare Event Search with Superconducting Thermometers dark matter detector experiment use calcium tungstate as a scintillator as well.[14] It is also utilized influorescent lighting systems for its ability to convertultraviolet light intovisible light.[15] In somecathode-ray tubes (CRTs), calcium tungstate (scheelite) is used as aphosphorescent screen material.[16]
Scheelite figures in the manga seriesDr. Stone, as a precursor to tungsten, and for its fluorescence.[17]
Ore minerals, mineral mixtures andore deposits | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Ores |
| ||||||||
| Deposit types | |||||||||
